1. Field of Invention
The invention relates generally to contraband detection systems and more specifically to x-ray inspection systems employed for contraband detection.
2. Discussion of Related Art
Contraband detection systems are used to rapidly detect contraband concealed in items under inspection. Such systems are used for security applications to detect weapons or explosives hidden in items such as luggage or cargo containers. Many types of contraband detection systems are in use.
Many contraband detection systems detect contraband by forming images using x-rays or, more generally, penetrating radiation, These systems contain a radiation source that emits radiation toward an item under inspection. A detector can array is used to measure the magnitude of the radiation after it has passed through the item under inspection. The measured radiation is used to compute properties of the item under inspection.
Different types of system may have different arrangements of the source and detectors or may process the detected radiation differently or compute different properties. For example, single view systems project radiation through the item under inspection in a single direction. An image formed with a single view system represents a projection of objects within the item into a single plane. In contrast, a computed tomography system (often called a “CT system”) uses a rotating source and rotating detector array to measure radiation passing through the item under inspection from multiple angles. These measurements are used to compute the density of small regions arrayed in a slice through the item under inspection.
Many systems are scanning systems, meaning that the item under inspection moves relative to the radiation source/detector combination. Often, the item under inspection moves on a belt past the source and detectors. Radiation measurements are taken at successive intervals as the item under inspection passes the detector arrays. Each set of measurements is used to create a portion of the image of the item under inspection. For example, in a single view system, the detector array is often a linear array that is transverse to the direction of motion of the belt. Each measurement taken with the detector array provides information for one line in an image. The successive measurements can be combined to form a projection of the entire item under inspection. In a CT system, multiple slices through an item under inspection can be combined to form a three dimensional model of the item.
CT systems used for contraband detection are called small-angle cone beam CT or alternatively fan beam CT, although this latter term is somewhat confusing as the term “fan” more typically refers to the beam angle along the detector array. Small-angle cone beam CT have divergences up to around +/−2 degrees, meaning that the beam may have an angle of about +/−2 degrees in a direction transverse to the axis of rotation of the source.
Small-angle cone beam CT systems typically contain detector arrays that are as wide as the beam at the point where the beam intersects the detector array. Wider arrays can be formed by including multiple rows of detectors in the array. Multiple rows of detectors allow data from which multiple slices can be constructed to be collected at one time. For example, previous CT's, such as the Examiner inspection system sold by the assignee of the present application, have a sampling width of about 13 cm, made up of 24 rows of detectors about 5 mm wide.
The data from the multiple rows of detectors is usually processed using algorithms that assume the radiation from the source to each of the rows of detectors in the array travels in parallel planes. If the beam diverges, this assumption is not exactly true, but so long as the divergence is relatively small, the deviation does not significantly impact the resultant image. Other processing techniques interpolate to measurements that would have been obtained had the rays traveled in parallel planes. So long as the divergence is small, such interpolations are sufficiently accurate. Large-angle cone beam CT systems, sometimes called simply cone beam CT systems, have been used in medical applications. Data processing techniques that do not rely on the assumption that radiation reaches all rows of detectors in parallel planes have been developed for these applications. For many years, the Feldkamp (FDK) method, as described in Feldkamp, L. A., L. C. Davis and J. W. Kress, “Practical cone-beam algorithm”, J. Opt Soc. of Am. 1(6), pp. 612-619 (1984) was considered the best choice for CBCT reconstruction. Advances made by Grangeat as described in Grangeat, P., “Mathematical framework of cone-beam 3D reconstruction via the first derivative of the Radon transform,” Mathematical Methods in Tomography, G. T. Herman, A. K. Louis and F. Natterer, eds., Lecture Notes in Mathematics, 1497, pp. 66-97, Springer-Verlag, Berlin (1991) established a new methodology toward exact reconstruction, linking the projection data to a derivative of 3-D Radon transform. Kasevitch succeeded in reducing this formalism to filtered back-projection technique that is both fast and accurate, as described in Kasevitch, A., “An improved exact filtered back-projection algorithm for spiral computed tomography,” Adv. Appl. Math. 32(4), pp. 681-697 (2004) and Kasevitch, A., “Theoretically exact FBP-type inversion algorithm for spiral CT,” SIAM Jour. Appl. Math., 62, pp. 2012-2026 (2002).